Oxidation of Fatty Acids:- The Four β-Oxidation Steps Are Repeated to Yield Acetyl-CoA and ATP
In one pass through the -oxidation sequence, one molecule of acetyl-CoA, two pairs of electrons, and four pro tons (H+) are removed from the long-chain fatty acyl–CoA, shortening it by two carbon atoms. The equation for one pass, beginning with the coenzyme A ester of our example, palmitate, is
Palmitoyl-CoA+CoA+FAD+NAD++H2O→myristoyl-CoA+acetyl-CoA+FADH2+NADH+H+ (17–2)
Following removal of one acetyl-CoA unit from palmitoyl CoA, the coenzyme A thioester of the shortened fatty acid (now the 14-carbon myristate) remains. The myristoyl-CoA can now go through another set of four β-oxidation reactions, exactly analogous to the first, to yield a second molecule of acetyl-CoA and lauroyl-CoA, the coenzyme A thioester of the 12-carbon laurate. Altogether, seven passes through the β-oxidation sequence are required to oxidize one molecule of palmitoyl-CoA to eight molecules of acetyl-CoA (Fig. 17–8b). The overall equation is
Palmitoyl-CoA+7CoA+7FAD+7NAD+7H2O →8 acetyl-CoA+7FADH2+7NADH+7H+ (17–3)
Each molecule of FADH2 formed during oxidation of the fatty acid donates a pair of electrons to ETF of the respiratory chain, and about 1.5 molecules of ATP are generated during the ensuing transfer of each electron pair to O2. Similarly, each molecule of NADH formed delivers a pair of electrons to the mitochondrial NADH de hydrogenase, and the subsequent transfer of each pair of electrons to O2 results in formation of about 2.5 molecules of ATP. Thus, four molecules of ATP are formed for each two-carbon unit removed in one pass through the sequence. Note that water is also produced in this process. Transfer of electrons from NADH or FADH2 to O2 yields one H2O per electron pair. Reduction of O2 by NADH also consumes one H+ per NADH molecule:
NADH+H++1/2 O2→NAD++ H2O.
In hibernating animals, fatty acid oxidation provides metabolic energy, heat, and water—all essential for survival of an animal that neither eats nor drinks for long periods (Box 17–1). Camels obtain water to supplement the meager supply available in their natural environment by oxidation of fats stored in their hump. The overall equation for the oxidation of palmitoyl CoA to eight molecules of acetyl-CoA, including the electron transfers and oxidative phosphorylations, is
Palmitoyl-CoA+7CoA+7O2+28Pi+28ADP→8 acetyl-CoA+28ATP+7H2O (17–4)
